The effects of magnetic fields on the lifetimes of radical pairs formed by photoinduced electron transfer and the corresponding yields of the products of radical pair decay have been studied for many years. (Steiner, U. E. et al. Chem. Rev. 1989, 89, 51-147.; Steiner, U. E. et al. Photochemistry and Photophysics, Vol IV; Rabek, J. F., Ed.; CRC: Boca Raton, Fla., 1991; pp 1-130.; Hayashi, H. In Photochemistry and Photophysics, Vol I; Rabek, J. F., Ed.; CRC: Boca Raton, Fla., 1990; pp 59-136.; Grissom, C. B. Chem. Rev. 1995, 95, 3-24). These effects arise because the field can affect the rates of interconversion among the singlet radical pair and the three sublevels of the triplet radical pair. The observation of such effects requires two apparently contradictory conditions. In order for photoinduced electron transfer to occur from an excited singlet state, the electronically excited electron donor (acceptor) and the ground-state acceptor (donor) must experience relatively strong electronic coupling so that electron transfer can compete kinetically with the other decay pathways available to the excited state. However, such strong coupling generally precludes the interconversion of the singlet and triplet radical pair states necessary for the development of magnetic field effects. Both conditions may be satisfied sequentially by allowing diffusional processes to bring together the donor and acceptor, thus promoting rapid photoinduced electron transfer. Diffusion can then separate the radical pairs, reducing coupling and allowing singlet-triplet interconversion. Thus, magnetic field effects are generally observable in radical pair systems wherein the donors and acceptors freely diffuse in solution, or in biradicals where the radicals are linked by flexible chains such as polymethylene groups so that large-scale internal motions are facile.
Because of these restrictions, rigid donor-acceptor assemblies, or those in media such as low-temperature glasses or plastics where molecular motions are restricted, typically do not demonstrate magnetic field effects on radical pairs originating from excited singlet state precursors. This hinders the use of such effects in the design of molecular-scale electronic components that must function in rigid media. The conundrum can be avoided by employing a multistep electron transfer strategy whereby the electron is moved from the primary donor to the ultimate acceptor via intermediate donor-acceptor species. In this way, the electronic coupling between adjacent donor-acceptor pairs is strong enough so that each electron transfer step is rapid and can compete with other decay pathways, resulting in a high yield of the final charge-separated state. At the same time, the electronic coupling between the donor and the ultimate acceptor is small, and this can allow rapid singlet-triplet interconversions and consequently magnetic field effects.
The preeminent example of this phenomenon is photosynthesis, where a number of different magnetic field effects on reaction yields and rates have been observed, (see: Blankenship, R. E. et al. Biochim. Biophys. Acta 1977, 461, 297-305.; Blankenship, R. E. Acc. Chem. Res. 1981, 14, 163-170.; Hoff, A. J. et al. Biochim. Biophys. Acta 1977, 460, 547-554.; Hoff, A. J. Photochem. Photobiol. 1986, 43, 727-745.; Boxer, S. G. et al. J. Am. Chem. Soc. 1982, 104, 1452-1454.; Boxer, S. G. et al. J. Am. Chem. Soc. 1982, 104, 2674-2675.; van Dijk, B. et al. J. Phys. Chem. B 1998, 102, 464-472). In the case of photosynthetic model systems, magnetic-field-dependent nonexponential decays of biradical states at room temperature have been reported in diporphyrin-imide triad molecules, (see: Werner, U. et al. J. Phys. Chem 1995, 99, 13930-13937). Small magnetic fields increased the initial rate of decay of the biradical to the ground state by charge recombination. Magnetic field effects have also been reported in dyads consisting of porphyrins linked to viologen electron acceptors by flexible chains. (Saito, T. et al. Bull. Chem. Soc. Jpn. 1988, 61, 1925-1931.; Nakamura, H. et al. Chem. Lett. 1 987, 543-546). In these reported cases, the photoinduced electron transfer originates from the porphyrin excited triplet state, rather than the singlet.
Recently reported (see: Liddell, P. A. et al. J. Am. Chem. Soc. 1997, 119, 1400-1405.; Carbonera, D. et al. J. Am. Chem. Soc. 1998, 120, 4398-4405; Gust, D. et al. In Recent Adavances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Rutherford, A. W., Eds.; The Electrochemical Society: Pennington, N.J., 1997; pp 9-24) is the preparation and study of a carotenoid (C) porphyrin (P) fullerene (C60) triad artificial photosynthetic reaction center (1), shown below, that demonstrates photo-induced electron transfer behavior ideally suited for the observation of unusual magnetic field effects. Excitation of the porphyrin moiety yields C-1P-C60, which decays by photoinduced electron transfer to give C-P+-C60*−. Secondary electron transfer from the carotenoid to the porphyrin radical cation produces the C*+-P-C60*− charge-separated state. This process occurs even in low-temperature organic glasses where molecular motions and some electron spin relaxation processes are slowed. In addition, charge recombination yields only the carotenoid triplet state, 3C-P-C60, rather than the molecular ground state. As discussed below, this combination of properties results in a lifetime for the C*+-P-C60*− charge-separated state on tile microsecond time scale that is increased by 50% upon application of a weak magnetic field. 